Small dogs display more aggressive behaviour than large dogs in social media videos

Due to potentially greater vulnerability to accidents and attacks, the behaviour of small dogs may reflect greater threat sensitivity and need for protection than that of large dogs. Based on this hypothesis, I predicted that dogs of small breeds (15 kg). I extracted behavioural data from 310 videos posted on YouTube depicting adult dogs of four small dog breeds (Chihuahua, Jack Russell Terrier, Dachshund and Yorkshire Terrier) and four large dog breeds (German Shepherd Dog, Border Collie, Labrador Retriever and Rottweiler; n=20 dogs/breed; mean±SE video duration: 59.8±2.0 s). Search terms included the breed name (n=160) for the control group, and the breed name with ‘angry’ (n=150) for the “angry” group. Behaviour in each video was scored using 1-0 sampling and effects of body size, breed, group and location were analysed by generalised linear models. Small dogs were more likely to show more total aggression (sum of vigilant, tail up, baring teeth, short bark, repeated barking, growling, snapping, biting: 1.9±0.13 vs 1.6±0.104; p=0.034), than large dogs. Snapping and biting incidents occurred in more of the small dogs, than the large dogs (0.22±0.05 vs 0.05±0.02; p=0.002). There were no differences between small and large dogs when it came to showing stress-related behaviour (sum of eye white, blinking, lip licking, trembling, panting, yawning, licking own body, ground sniffing, scratching), submissive behaviour (sum of looking away, withdrawing, tail down, paw lifting, presenting belly), and attention-seeking behaviour directed towards the handler (Sum of face licking, whimpering, paws on body, play invite, body licking, tail wagging, jumping). Small dogs were more likely to be on an elevated surface (including being held in a person’s arms) than large dogs (0.35±0.06 vs 0.08±0.03; p=